Electron discharge apparatus



Sept. 3, 1946.

'J. H. FREM LIN- ELECTRON DISCHARGE APPARATUS Filed June 25, 1941 Fig.1;

Ina/enter Attorney Patented Sept. 3, 1946 ELECTRON DISCHARGE APPARATUSLondon, England, assignor John Heaver Fremlin,

to International Stan New York, N. Y.

dard Electric Corporation,

Application June 25, 1941, Serial No. 399,598

In Great 6 Claims.

This invention relates to electron discharge apparatus incorporatingfocussing systems for electron beams set up in the apparatus.

It is common practice to provide, as focussing electrodes for anelectron tured-disc and other-shaped metal bodies at different pointsalong the path of the beam such that desired beam converging fields andbeam diverging fields may be set up mainly in the gaps between the bodieby the application of different potentials to the bodies.

Proposals have also been made, particularly with a view to obtainingcontinuous potential gradients along substantial lengths of the beampath and thereby facilitating the concentration of beams of largeintensity and density, to provide interleaved conductors of differentpotential in a common cylindrical surface, the conductors atonepotential being tapered in one direction and the conductors of theother potential being tapered in the opposite direction along the axis.

The present invention provides alternative structures providing greaterfreedom in design and simplicity in manufacture.

The structure now proposed comprises a foraminate tube or tubular gridsurrounded by a co-axial tube capable of influencing the field withinthe foraminate tube or tubular grid, one or both of said tubularelements being of nonuniform diameter along the length of the systemPreferably we employ a tubular grid of uniform diameter and a flaredouter tube. The grid may be a wire helix or it may be constructed ofparallel wires in a cylindrical boundary. The outer tube is preferablyof sheet metal although it may be foraminate so long as the mesh is finecompared with that of the tubular grid.

A focussing structure embodying the invention is shown in theaccompanying drawing, Fig. 2 of which shows a section along the line A-Aof Fig. 1. The structure shown comprises a tu bular grid of parallelwires F extending from a metal tube E. Surrounding the tubular grid is asheet metal tube G converging from a large diameter end adjacent thetube E to a cylindrical portion 13 of diameter comparable with the griddiameter at the free end of the grid wires F. A transverse insulatingannulus C is provided for the mechanical support of the grid wires.

The tubular grid with its tubular extension E and the outer sheet metaltube G are adapted to be maintained at difierent potentials.

It will be seen from the figure that initially electrons coming frombelow as indicated by arrows H are moving in a space at a potentialsenbeam, tubular, aper- Britain June 21, 1940 sibly equal to that of theconductor B. They then pass through the space within the grid wires inwhich the potential changes continuously. Finally they emerge into theequi-potential space within metal tube E. The potential gradient withinthe grid depends on the curvature of the wall of the outer conductor Gand can be made of any desired form by suitably choosing this curvature.

In the figure the potential of the space within metal tube E is madeequal to that of the grid. There is no reason, however, why the diameterand potential of DE should not have any desired arbitrary value. In thesame way the potential of the initial tubular portion B may be at anyarbitrary potential. In some cases it is convenient to have the diameterand potential of B or E intermediate between those of the grid and ofthe continuous external conductor, the grid wires being then supportedat both ends for example by thin sheets of mica.

From consideration of the conditions obtaining in triodes it is knownthat when a grid at potential Vg is placed near to a plate at adifferent potential Vp then the grid will behave in some respects asthough an effective potential V existed over the whole surface definedby the grid, where V depends both on V and Vp, on the mechanicalstructure of the grid, 1. e. on wire size and grid pitch and also on thedistance apart of the grid and plate. The present invention makes use ofthe property that the effective potential in a grid surface can bevaried continuously in any desired manner by suitably shaping the outerrelatively unbroken conductor and by suitably arranging thecharacteristics of the grid. Thus a convenient means is provided for theconstruction of certain types of electron gun.

Although in the figure the grid is shown as of uniform diameter, theouter conductor being flared, similar efiects can be secured by using anouter tube of uniform diameter and a flared grid. Alternatively bothelements may be of non-uniform diameter.

It will be clear that any desired potential distribution along theelectron path can be set up by suitable grading of the tube diametersalong the path, choice of grid aperture and of potentials.

In some cases it is desirable to arrange that the varying diameterchanges in different senses at different points along the beam path. Incases where in the past three or more successive tubular or aperturedelectrodes each requiring a different potential have been employed,equivalent field conditions can be now set up by the use of but twoelectrodes of different potential. The shape of the electrodes can bearranged so that specified intermediate potentials appear at chosenpoints along the beam path.

What i claimed is:

1. Electron discharge apparatus comprising, as a focussing systemadapted When suitable potentials are applied to provide a desiredpotential gradient along the path of an electron beam, a foraminatetubular electrode surrounded by a coaxial tube capable of influencingthe field within the foraminate tubular electrode, said electrode andtube being coextensive throughout at least part of their length and atleast one of said tubular elements being of non-uniform diameter alongthe coextensive length of the ystem.

2. Electron discharge apparatus comprising, as a focussing systemadapted to provide a desired potential gradient along the path of anelectron beam, a foraminate tubular grid of substantially uniformdiameter a coaxial tube surrounding said foraminate tubular grid andextending at least in part along the length of said grid and being ofvarying diameter along said coextensive part adapted when given apotential different from that of the foraminate tubular grid to modifythe field within said foraminate tube or tubular grid.

3. Electron discharge apparatus according to claim 2 wherein parallelwires in a cylindrical boundary serve as tubular grid and a flared sheetmetal tube constitutes the outer co-axial tube.

4. Electron discharge apparatus according to claim 2 wherein theforaminate tubular grid has a sheet metal tubular extension at a flaredend of the outer tube.

5. Electron discharge apparatus according to claim 2 wherein the outertube converges upon and extends beyond one end of the foraminate tubulargrid.

6. Electron discharge apparatus as claimed in claim 2 comprising meanfor applying to the inner and outer tubes of the focussing systempotentials differing from each other and from that of a thermioniccathode disposed on the axis of the focussing. system and co-operatingwith said system to form an electron gun.

JOHN HEAVER FREMLIN.

